Great questions. I have 2 answers for you:
1) Sepp Holzer and a lot of people want to mimic nature. In nature, the seed will fall to the ground - some on good soil, some gets eaten - sounds like the parable Jesus taught. Not sure what Holzer would plant by seed, but I would guess about every kind of plant. The ones best for the situation will grow and thrive. Others just don't make it. That is what nature does so we should do that also.
2) I am not Sepp Holzer or a lot of other people. If I spend money on vegetable seeds, I want them to grow and produce so I am going to plant carefully and tend to the plants. As a result, I have to deal with sicker, weaker plants.
I do think the ultimate goal is to have seeds that are adapted to my specific property/area/microclimates. If I let them develop over the years, I will have good quality and abundance of seeds. Then I can just throw them on the ground like Sepp Holzer and have a paradise. Right now, I am just not there with my time or my resources.